Gas generant and manufacturing method thereof

ABSTRACT

The present invention generally relates to gas generant compositions for inflators of occupant restraint systems, for example. An extrudable pyrotechnic composition includes polyvinylazoles for use within an airbag gas generator. The fuel may be selected from exemplary polyvinylazoles including 5-amino-1-vinyltetrazole, poly(5-vinyltetrazole), poly(2-methyl-5-vinyl) tetrazole, poly(1-vinyl) tetrazole, poly(3-vinyl) 1,2,5 oxadiazole, and poly(3-vinyl) 1,2,4-triazole. An oxidizer is combined with the fuel and preferably contains phase stabilized ammonium nitrate. A novel method of forming the compositions is also presented wherein the various constituents are wetted and/or dissolved, and then cured within the polyvinylazole matrix thereby forming a more intimate combination within the gas generant composition. A vehicle occupant protection system 180, and other gas generating systems, incorporate the compositions of the present invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a continuation of co-pendingand co-owned U.S. application Ser. No. 11/092,377 having a filing dateof Mar. 29, 2005, which claimed the benefit of U.S. ProvisionalApplication Ser. No. 60/557,279 filed on Mar. 29, 2004.

TECHNICAL FIELD

The present invention relates generally to gas generating systems, andto gas generant compositions employed in gas generator devices forautomotive restraint systems, for example.

BACKGROUND OF THE INVENTION

The present invention relates to nontoxic gas generating compositionsthat upon combustion rapidly generate gases that are useful forinflating occupant safety restraints in motor vehicles and specifically,the invention relates to thermally stable nonazide gas generants havingnot only acceptable burn rates, but that also, upon combustion, exhibita relatively high gas volume to solid particulate ratio at acceptableflame temperatures.

The evolution from azide-based gas generants to nonazide gas generantsis well-documented in the prior art. The advantages of nonazide gasgenerant compositions in comparison with azide gas generants have beenextensively described in the patent literature, for example, U.S. Pat.Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and5,035,757, the discussions of which are hereby incorporated byreference.

In addition to a fuel constituent, pyrotechnic nonazide gas generantscontain ingredients such as oxidizers to provide the required oxygen forrapid combustion and reduce the quantity of toxic gases generated, acatalyst to promote the conversion of toxic oxides of carbon andnitrogen to innocuous gases, and a slag forming constituent to cause thesolid and liquid products formed during and immediately after combustionto agglomerate into filterable clinker-like particulates. Other optionaladditives, such as burning rate enhancers or ballistic modifiers andignition aids, are used to control the ignitability and combustionproperties of the gas generant.

One of the disadvantages of known nonazide gas generant compositions isthe amount and physical nature of the solid residues formed duringcombustion. When employed in a vehicle occupant protection system, thesolids produced as a result of combustion must be filtered and otherwisekept away from contact with the occupants of the vehicle. It istherefore highly desirable to develop compositions that produce aminimum of solid particulates while still providing adequate quantitiesof a nontoxic gas to inflate the safety device at a high rate.

The use of phase stabilized ammonium nitrate as an oxidizer, forexample, is desirable because it generates abundant nontoxic gases andminimal solids upon combustion. To be useful, however, gas generants forautomotive applications must be thermally stable when aged for 400 hoursor more at 107.degree. C. The compositions must also retain structuralintegrity when cycled between −40.degree. C. and 107.degree. C. Further,gas generant compositions incorporating phase stabilized or pureammonium nitrate sometimes exhibit poor thermal stability, and produceunacceptably high levels of toxic gases, CO and NO.sub.x for example,depending on the composition of the associated additives such asplasticizers and binders.

Yet another problem that must be addressed is that the U.S. Departmentof Transportation (DOT) regulations require “cap testing” for gasgenerants. Because of the sensitivity to detonation of fuels often usedin conjunction with ammonium nitrate, many propellants incorporatingammonium nitrate do not pass the cap test unless shaped into largedisks, which in turn reduces design flexibility of the inflator.

Yet another concern includes slower cold start ignitions of typicalsmokeless gas generant compositions, that is gas generant compositionsthat result in less than 10% of solid combustion products.

Yet another concern regards the environmental impact of manufacturinggas generant compositions. In many manufacturing processes, the gasgenerant is formed in a solvent-based process. As such, the organicsolvent remnant must be disposed of with the attendant environmentalconcerns.

Accordingly, ongoing efforts in the design of automotive gas generatingsystems, for example, include other initiatives that desirably producemore gas and less solids without the drawbacks mentioned above.

SUMMARY OF THE INVENTION

The above-referenced concerns are resolved by gas generating systemsincluding a gas generant composition containing an extrudablepolyvinylazole fuel such as a polyvinyltetrazole, polyvinyltriazole, orpolyvinyldiazole. Preferred oxidizers include nonmetal oxidizers such asammonium nitrate and ammonium perchlorate. Other oxidizers includealkali and alkaline earth metal nitrates.

The fuel is selected from the group of polyvinyltetrazoles,polyvinyltriazoles, polyvinyldiazoles or polyvinylfurazans, and mixturesthereof. A preferred group of fuels includes polymeric tetrazoles,triazoles, and oxadiazoles (furazans), having functional groups on theazole pendants. Although compositions containing NH₃ linkages andcarbon/hydrogen content are generally useful, preferred compositionswill not contain NH₃ linkages due to handling concerns, and the carbonand hydrogen content will be minimized to inhibit the formation ofcarbon dioxide and water. Preferred vinyl tetrazoles include5-Amino-1-vinyltetrazole and poly(5-vinyltetrazole), both exhibitingself-propagating thermolysis or thermal decomposition. Other fuelsinclude poly(2-methyl-5-vinyl) tetrazole, poly(1-vinyl) tetrazole,poly(3-vinyl) 1,2,5-oxadiazole, and poly(3-vinyl) 1,2,4-triazole. Theseand other possible fuels are structurally illustrated in the figuresincluded herewith. The fuel preferably constitutes 10-40% by weight ofthe gas generant composition.

An oxidizer is preferably selected from the group of nonmetal, andalkali and alkaline earth metal nitrates, and mixtures thereof. Nonmetalnitrates include ammonium nitrate and phase stabilized ammonium nitrate,stabilized as known in the art. Alkali and alkaline earth metal nitratesinclude potassium nitrate and strontium nitrate. Other oxidizers knownfor their utility in air bag gas generating compositions are alsocontemplated. The oxidizer preferably constitutes 60-90% by weight ofthe gas generant composition.

Other gas generant constituents known for their utility in air bag gasgenerant compositions may be employed in effective amounts in thecompositions of the present invention. These include, but are notlimited to, coolants, slag formers, and ballistic modifiers known in theart.

In sum, the present invention includes gas generant compositions thatmaximize gas combustion products and minimize solid combustion productswhile retaining other design requirements such as thermal stability.These and other advantages will be apparent upon a review of thedetailed description.

In yet another aspect of the invention, a method of manufacturing a gasgenerant composition incorporating a polyvinylazole is described. Avinyl azole is first added to a vessel. If necessary, an aqueous,organic, or aqueous/organic solvent is provided in an amount effectiveto dissolve all constituents to be added to the vessel. In preferredembodiments, a liquid vinyl azole will complete wet and/or facilitatethe solubility of the other gas generant constituents without the use ofa solvent. An oxidizer, preferably nonmetallic, is next added. Otherconstituents/solutes such as a secondary fuel(s), a secondaryoxidizer(s), slag former(s), processing aid(s), coolant(s), and/or burnrate modifier(s) may be added to the slurry and stirred to asubstantially uniform or homogeneous mixture. Next, an initiator isadded to facilitate the curing or polymerization of the mixture. Themixture is then cured either statically, or without stirring, wherein asolid is then formed, or, while stirring wherein granules may then beformed. If cured statically, the mixture may be poured within molds, forexample, to form the desired propellant shape(s). If cured whilestirring, crushing and formation of the granules is not necessary giventhe inherent formation of the granules. Thermoplastic polymersfacilitate melt processing for further shaping of the propellant ifdesired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary airbag inflator containing a gas generantcomposition formed in accordance with the present invention.

FIG. 2 is a schematic representation of an exemplary vehicle occupantrestraint system incorporating the inflator of FIG. 1 and a gas generantin accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention generally relates to gas generant compositions forinflators of occupant restraint systems. In accordance with the presentinvention, a pyrotechnic composition includes extrudable fuels such aspolyvinyltetrazoles (PVT) for use within a gas generating system, suchas that exemplified by a high gas yield automotive airbag propellant ina vehicle occupant protection system. The fuel also functions as abinder. Preferred oxidizers include nonmetal oxidizers such as ammoniumnitrate and ammonium perchlorate. Other oxidizers include alkali andalkaline earth metal nitrates.

The fuel is selected from the group of polyvinyltetrazoles,polyvinyltriazoles, polyvinyldiazoles or polyvinylfurazans, and mixturesthereof. A preferred group of fuels includes polymeric tetrazoles,triazoles, and oxadiazoles (furazans), having functional groups on theazole pendants. Although compositions containing HN₃ linkages andcarbon/hydrogen content are generally useful, preferred compositionswill not contain HN₃ linkages due to handling concerns, and the carbonand hydrogen content will be minimized to inhibit the formation ofcarbon monoxide, carbon dioxide, and water. In general, the consumptionof oxygen from the oxidizer is preferentially inhibited with regard tothe formation of these gaseous or vapor products. Preferred vinyltetrazoles include 5-Amino-1-vinyltetrazole and poly(5-vinyltetrazole),both exhibiting self-propagating thermolysis or thermal decomposition.Other fuels include poly(2-methyl-5-vinyl) tetrazole, poly(1-vinyl)tetrazole, poly(3-vinyl) 1,2,5-oxadiazole, and poly(3-vinyl)1,2,4-triazole. These and other possible fuels are exemplified by, butnot limited to, the structures shown below.

As such, it has been discovered that an additional benefit with thepresent fuels is that compositions resulting in difficult cold-startignitions that necessitate more powerful ignition trains and boosters,are avoided. Poly(5-amino-1-vinyl) tetrazole, for example, has noendothermic process before exothermic decomposition begins. Therefore,the heat-consuming step normally attendant prior to the energy releasingsteps of combustion (that acts as an energy barrier) is not present inthe present compositions. It is believed that other polymeric azolesfunctioning as fuels in the present invention have the same benefit. Thepolyvinylazole fuel preferably constitutes 5-40% by weight of the gasgenerant composition.

An oxidizer is preferably selected from the group of nonmetal, andalkali and alkaline earth metal nitrates, and mixtures thereof. Nonmetalnitrates include ammonium nitrate and phase stabilized ammonium nitrate,stabilized as known in the art. Alkali and alkaline earth metal nitratesinclude potassium nitrate and strontium nitrate. Other oxidizers knownfor their utility in air bag gas generating compositions are alsocontemplated. The oxidizer preferably constitutes 60-95% by weight ofthe gas generant composition.

Other gas generant constituents known for their utility in air bag gasgenerant compositions may be employed in effective amounts in thecompositions of the present invention. These include, but are notlimited to, coolants, slag formers, and ballistic modifiers known in theart.

The gas generant constituents of the present invention are supplied bysuppliers known in the art and are preferably blended by a wet method. Asolvent chosen with regard to the group(s) substituted on the polymericfuel is heated to a temperature sufficient to dissolve the fuel butbelow boiling, for example just below 100° C., but low enough to preventautoignition of any of the constituents as they are added and then laterprecipitate. Hydrophilic groups, for example, may be more efficientlydissolved by the use of water as a solvent. Other groups may be moreefficiently dissolved in an acidic solution, nitric acid for example.Other solvents include alcohols and plasticizers such as polyethyleneglycol. Once a suitable solvent is chosen and heated, the fuel is slowlyadded and dissolved. The oxidizer is then slowly added and alsodissolved. Any other desirable constituents are likewise dissolved. Thesolution is heated and continually stirred. As the solvent is cooked offover time, the fuel and oxidizer, and any other constituents, areco-precipitated in a homogeneous solid solution. The precipitate isremoved from the heat once the solvent has been at least substantiallyvolatilized, but more preferably completely volatilized. The compositionmay then be extruded into pellets or any other useful shape.

The polymeric fuels may be manufactured by known processes. For example,vinylation of a tetrazole with vinyl acetate, followed by polymerizationis described in Vereshchagin, et al., J. Org. Chem. USSR (Engl. Transl.)22(9), 1777-83, (1987). The synthesis of various vinyltetrazoles is alsodescribed in Russian Chemical Reviews 72(2), pages 143-164 (2003),herein incorporated by reference. The methyl-group of the startingtetrazole can be exchanged for an amino group. The vinyltetrazoles arethen polymerized using a common polymerization initiator such asazoisobutyronitrile (AIBN). It is believed that similar vinylation offurazans and triazoles will also yield the polyvinyldiazoles andpolyvinyltriazoles of the present invention. Exemplary reactions givenbelow illustrate how various polyvinyldiazoles, polyvinyltriazoles andpolyvinyltetrazoles may be formed. Reaction 1 illustrates howpolyvinyldiazoles may be formed. Reaction 2 illustrates howpolyvinyltriazoles may be formed. Reaction 3 exemplifies howpolyvinyltetrazoles may be formed.

A generic polyvinylazole, or a structure that generically represents thepolyvinyltetrazoles, polyvinyltriazoles, and polyvinyldiazoles of thepresent invention, may be represented by an aromatic ring having fivecites that contains,

Stated another way, the aromatic ring will contain from zero to a singleoxygen atom, will contain at least two nitrogen atoms, and will containat least one carbon atom. More preferably, a gas generant composition ofthe present invention will contain a polymeric azole and phasestabilized ammonium nitrate. The advantages are high gas yield and lowsolids production, a high energy fuel/binder, and a low-cost oxidizerthereby obviating the need for filtration of the gas given that littleif any solids are produced upon combustion. The compositions of thepresent invention may be extruded given the pliant nature of thepolymeric fuels.

The gas generant compositions of the present invention may also containa secondary fuel formed from amine salts of tetrazoles and triazoles.These are described and exemplified in co-owned U.S. Pat. Nos.5,872,329, 6,074,502, 6,210,505, and 6,306,232, each herein incorporatedby reference. The total weight percent of both the first and secondfuels, or the fuel component of the present compositions, is about 10 to40 weight % of the total gas generant composition.

More specifically, nonmetal salts of tetrazoles include in particular,amine, amino, and amide salts of tetrazole and triazole selected fromthe group including monoguanidinium salt of 5,5′-Bis-1H-tetrazole(BHT.1GAD), diguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.2GAD),monoaminoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.1AGAD),diaminoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.2AGAD),monohydrazinium salt of 5,5′-Bis-1H-tetrazole (BHT.1HH), dihydraziniumsalt of 5,5′-Bis-1H-tetrazole (BHT.2HH), monoammonium salt of5,5′-bis-1H-tetrazole (BHT.1NH₃), diammonium salt of5,5′-bis-1H-tetrazole (BHT.2NH₃), mono-3-amino-1,2,4-triazolium salt of5,5′-bis-1H-tetrazole (BHT.1ATAZ), di-3-amino-1,2,4-triazolium salt of5,5′-bis-1H-tetrazole (BHT.2ATAZ), and diguanidinium salt of5,5′-Azobis-1H-tetrazole (ABHT.2GAD).

Amine salts of triazoles include monoammonium salt of3-nitro-1,2,4-triazole (NTA.1NH₃), monoguanidinium salt of3-nitro-1,2,4-triazole (NTA-1 GAD), diammonium salt of dinitrobitriazole(DNBTR.2NH₃), diguanidinium salt of dinitrobitriazole (DNBTR.2GAD), andmonoammonium salt of 3,5-dinitro-1,2,4-triazole (DNTR.1 NH₃).

A generic nonmetal salt of tetrazole as shown in Formula I includes acationic nitrogen containing component, Z, and an anionic componentcomprising a tetrazole ring and an R group substituted on the 5-positionof the tetrazole ring. A generic nonmetal salt of triazole as shown inFormula II includes a cationic nitrogen containing component, Z, and ananionic component comprising a triazole ring and two R groupssubstituted on the 3- and 5-positions of the triazole ring, wherein R₁may or may not be structurally synonymous with R₂. An R component isselected from a group including hydrogen or any nitrogen-containingcompound such as an amino, nitro, nitramino, or a tetrazolyl ortriazolyl group as shown in Formula I or II, respectively, substituteddirectly or via amine, diazo, or triazo groups. The compound Z issubstituted at the 1-position of either formula, and is formed from amember of the group comprising amines, aminos, and amides includingammonia, carbohydrazide, oxamic hydrazide, and hydrazine; guanidinecompounds such as guanidine, aminoguanidine, diaminoguanidine,triaminoguanidine, dicyandiamide and nitroguanidine; nitrogensubstituted carbonyl compounds or amides such as urea, oxamide,bis-(carbonamide) amine, azodicarbonamide, and hydrazodicarbonamide;and, amino azoles such as 3-amino-1,2,4-triazole,3-amino-5-nitro-1,2,4-triazole, 5-aminotetrazole,3-nitramino-1,2,4-triazole, 5-nitraminotetrazole, and melamine.

Example 1

A gas generant composition of the present invention is formed by firstsynthesizing a polyvinyltetrazole. A generic substituted tetrazole andvinyl acetate are combined to vinylate the tetrazole. The vinylatedtetrazole is added to a molar equivalent of mercury acetate and borontrifluoride-etherate for polymerization thereof. The resulting productsmay then be separated by oil distillation for example. Thepolyvinyltetrazoles illustrated in the drawings may be formed in thesame way. Reaction 3 exemplifies the process described above.

Example 2

A gas generant composition of the present invention is formed by firstsynthesizing a polyvinyltriazole. A generic substituted triazole metalor nonmetal salt is added to a molar equivalent amount of a free radicalbrominating reagent such as n-bromo-succinamide and to abenzoyl-peroxide free radical initiator to form a brominated triazole.The brominated triazole is then added to triphenyl phosphine to form aWittig salt group on the substituted triazole salt. The triazole salt isthen added to a metal or nonmetal organic or inorganic base, and also toformaldehyde to form a vinylated triazole salt. The vinylated triazolesalt is next added to a free radical polymerization reagent such asazoisobutyronitrile and a catalytic amount of a cationic polmerizer orZiegler-Natta catalyst such as a metal or titanium complex. Reaction 2exemplifies the process described above wherein the synthesis ofpolyvinyl-1,2,4-triazole) is described.

Example 3

A gas generant composition of the present invention is formed by firstsynthesizing a polyvinyldiazole. An alkenol containing two —OH groups isadded to acetic anhydride to form a substituted diazole. The substituteddiazole is then added to a molar equivalent amount of a free radicalbrominating reagent such as n-bromo-succinamide and to a free radicalinitiator such as benzoyl-peroxide to form a brominated diazole. Thesubstituted diazole is then added to triphenyl phosphine to form aWittig salt group on the substituted diazole salt. The diazole salt isthen added to a metal or nonmetal organic or inorganic base, and also toformaldehyde to form a vinylated diazole salt. The vinylated diazolesalt is next added to a free radical polymerization reagent such asazoisobutyronitrile and a catalytic amount of a cationic polymerizer orZiegler-Natta reagent such as a metal complex. Reaction 1 exemplifiesthe process described above wherein the synthesis ofpoly(vinyl-1,2,5-oxadiazole) is described.

Examples 4-9

Examples 4-9 are tabulated below and provide a comparative view of thedifferent types and amounts of gas produced with regard to several knowngas generant compositions and a gas generant formed in accordance withthe present invention. Example 4 is a representative gas generantcomposition formed from 5-aminotetrazole and strontium nitrate, inaccordance with U.S. Pat. No. 5,035,757 herein incorporated byreference. Example 5 is a representative gas generant composition formedfrom an amine salt of tetrazole such as diammonium salt of5,5′-bi-1H-tetrazole, phase stabilized ammonium nitrate, strontiumnitrate, and clay in accordance with U.S. Pat. No. 6,210,505 hereinincorporated by reference. Example 6 is a representative gas generantcomposition formed from an amine salt of tetrazole such as diammoniumsalt of 5,5′-bi-1H-tetrazole and phase stabilized ammonium nitrate inaccordance with U.S. Pat. No. 5,872,329 herein incorporated byreference. Example 7 is a representative gas generant composition formedfrom ammonium nitramine tetrazole and phase stabilized ammonium nitratein accordance with U.S. Pat. No. 5,872,329 herein incorporated byreference. Example 8 is a representative gas generant composition formedfrom ammonium nitramine tetrazole, phase stabilized ammonium nitrate,and a slag former in accordance with U.S. Pat. No. 5,872,329 hereinincorporated by reference. Example 9 is a representative compositionformed in accordance with the present invention containing ammoniumpolyvinyl tetrazole and phase stabilized ammonium nitrate (ammoniumnitrate coprecipitated with 10% potassium nitrate).

Table 1 details the relative amounts produced (ppm) of carbon monoxide(CO), ammonia (NH₃), nitrogen monoxide (NO), and nitrogen dioxide (NO₂)with regard to each example and the amount of gas generant in grams(Gg). All examples were combusted in a gas generator of substantiallythe same design.

TABLE 1 Example Gg P_(c) CO NH3 NO NO2 4 45 15 125 10 49 9 5 25 36 10965 29 4 6 25 29 111 29 37 5 7 25 36 62 10 28 3 8 25 37 98 35 33 4 9 2534 129 4 28 4

The data collected indicates that the composition of Example 9, formedin accordance with the present invention, results in far less ammoniathan the other examples, well below the industry standard of 35 ppm. Ithas been discovered that compositions of the present invention result insubstantially less amounts of ammonia as compared to other known gasgenerants. In many known gas generant compositions, it is oftendifficult to reduce the total amount of ammonia produced uponcombustion, even though other performance criteria remain favorable.

Examples 10-14

Theoretical examples 10-14 are tabulated below and provide a comparativeview of the different amounts and types of gas produced with regard toseveral gas generant compositions formed in accordance with the presentinvention. All phase stabilized ammonium nitrate (PSAN10) referred to inTable 2 has been stabilized with 10% by weight potassium nitrate of thetotal PSAN. All examples employ ammonium poly(C-vinyltetrazole) (APV) asthe primary fuel. Certain examples employ nonmetal diammonium salt of5,5′-Bis-1H-tetrazole (BHT.2NH₃) as a secondary fuel. All examplesreflect results generated by combustion of the gas generant constituents(propellant composition) within a similarly designed inflator or gasgenerator with equivalent heat sink design.

TABLE 2 Flame Gas Constituents Temp. Exhaust Combustion Example (wt % of100 g) (K) Temp. (K) Products (mol) 10 15% APV 2222  857 2.25 H2O 85%PSAN10 1.33 N2 0.39 CO2 11 16% APV 2057  900 2.25 H2O 40% PSAN10 1.33 N210% Strontium Nitrate 0.39 CO2 05% Clay 12 22% APV 2054 1225 0.64 H2O73% Strontium Nitrate 0.83 N2 05% Clay 0.52 CO2 13 08% APV 2036  8741.86 H2O 64.60% PSAN10 1.34 N2 10% Strontium Nitrate 0.35 CO2 05% Clay12.40% BHT.2NH3 14 08% APV 2206  835 2.20 H2O 80.60% PSAN10 1.45 N211.40% BHT.2NH3 0.34 CO2

Example 10 has been found to be thermally stable at 105 degrees Celsiusfor 400 hours with only a 0.5% mass loss. Accordingly, Example 10exemplifies the unexpected thermal stability of gas generantcompositions of the present invention, particularly those incorporatinga polyvinylazole as defined herein and phase stabilized ammonium nitrate(stabilized with 10% potassium nitrate). It should be emphasized thatother phase stabilizers are also contemplated as known or recognized inthe art.

Examples 11 through 13 exemplify the use of a polyvinylazole withmetallic oxidizers. In certain applications, the use of a metallicoxidizer may be desired for optimization of ignitability, burn rateexponent, gas generant burn rate, and other design criteria. Theexamples illustrate that the more metallic oxidizer is used the lessmols of gas produced upon combustion.

In contrast, Examples 10 and 14 illustrate that molar amounts of gascombustion products are maximized when nonmetal gas generantconstituents are employed. Accordingly, preferred gas generantcompositions of the present invention contain at least onepolyvinylazole as a fuel component and a nonmetal oxidizer as anoxidizer component.

Finally, with regard to Example 14, it has been found that the gasgenerant burn rate may be enhanced by adding another nonmetal fuel,BHT.2NH₃, to APV and PSAN10, thereby optimizing the combustion profileof the gas generant composition. The burn rate of Example 14 is recordedat 1.2 inches per second at 5500 psi. It can be concluded therefore,that the addition of nonmetal amine salts of tetrazoles and/or nonmetalamine salts of triazoles as described in 5,872,329 may be advantageouswith regard to burn rate and gas generation. Furthermore, the pliantnature of the APV provides extrudability of the propellant composition.

Examples 15 and 16

Examples 15 and 16 exemplify the cold start advantage of gas generantcompositions containing a polyvinylazole. As shown by differentialscanning calorimetry (DSR), typical smokeless or nonmetal compositionsmay exhibit an endothermic trend prior to exothermic combustion. As aresult, relatively greater amounts of energy must be available to ignitethe gas generant and sustain combustion of the same. Oftentimes, a moreaggressive ignition train, to include an aggressive booster compositionperhaps, is required to attain the energy level necessary to ignite thegas generant and sustain combustion. Example 15 pertains to acomposition containing 65% PSAN10 and about 35% BHT.2NH₃. As shown inFIG. 1, an endotherm is maximized at 253.12 degrees Celsius, therebyrepresenting a recorded loss of about 508.30 joules/gram of gasgenerant. In comparison, Example 16 pertains to a composition containingabout 15% poly(C-vinyltetrazole) and about 85% PSAN10. Mostunexpectedly, there is no endothermic process and accordingly,combustion proceeds in an uninhibited manner. As a result, less energyis required to combust the gas generant composition thereby reducing theignition train or ignition and booster requirements.

In yet another aspect of the invention, the present compositions asexemplified herein may be employed within a gas generating system. Forexample, as schematically shown in FIG. 2, a vehicle occupant protectionsystem made in a known way contains crash sensors in electricalcommunication with an airbag inflator in the steering wheel, and alsowith a seatbelt assembly. The gas generating compositions of the presentinvention may be employed in both subassemblies within the broadervehicle occupant protection system or gas generating system. Morespecifically, each gas generator employed in an automotive gasgenerating system may contain a gas generating composition as describedherein.

In yet another aspect of the invention, a method of manufacturing a gasgenerant composition includes polymerizing a monomer component of apolymeric binder/fuel in the presence of at least an oxidizer therebyforming a homogeneous solid composite gas generant formulation. Thepolymeric binder/fuel is generally selected from a myriad of polymericazoles including vinyl tetrazoles, vinyl triazoles, vinyl oxadiazoles(furazans), copolymers thereof, as described above for example.Functional groups may be present on the azole pendants, however,preferred compositions avoid HN₃ linkages due to sensitivity issues.Furthermore, preferred compositions will also have relatively loweramounts of carbon/hydrogen content thereby facilitating coolerformulations upon combustion believed attributable to the lower amountsof water and carbon dioxide formed. The polymeric binder/fuel isexemplified by any of several polyvinyl tetrazole compounds includingpoly(vinyl-5-amino)tetrazole, poly(vinyl-5-methyl)tetrazole,poly(5-amino-1-vinyltetrazole), poly(5-vinyltetrazole) andpoly(vinyl-bitetrazolamine), or mixtures thereof. Other polymeric azolefuels are illustrated in the discussion given above. Other fuelscontemplated as useful in the present invention include metal salts andcomplexes of the azole polymers described above.

The polymeric azoles may be purchased from suppliers known in the art.They may also be manufactured by vinylation of an azole with vinylacetate. For example, the vinylation of a tetrazole with vinyl acetate,followed by polymerization, yields desirable poly vinyl tetrazoles.

The procedure is detailed in Vereshchagin, et al., J. Org. Chem. USSR(Engl. Transl.), 22(9), 177-83, (1987), herein incorporated byreference. The methyl-group of the starting tetrazole may be exchangedfor an amino-group. The vinyltetrazoles are then polymerized using acommon polymerization initiator such as azoisobutyronitrile (AIBN).Other syntheses may be employed. It is believed that vinylation offurazans and triazoles would similarly yield desirable polymers.

The oxidizer is preferably selected from exemplary compounds to includealkali metal, alkaline earth metal, transitional metal, and nonmetalnitrates and perchlorates. Specific oxidizers include ammonium nitrate,phase stabilized ammonium nitrate, potassium nitrate, strontium nitrate,potassium perchlorate, ammonium perchlorate, sodium nitrate, sodiumperchlorate, and mixtures thereof.

When preparing the compositions, the monomer(s)/copolymer(s) andoxidizer(s) are added to an inorganic solvent such as water, or to anorganic solvent such as dimethylformamide, depending on the chemistry ofthe monomer/copolymers. If water is used, a water-based polymerizationinitiator such as ammonium persulfate is employed in the aqueous slurryresulting in a relatively thinner or less rigid slurry. If anorganic-based solvent is used, an organic solvent based initiator suchas azobisisobutyronitrile (AIBN) is employed in the organic slurryresulting in relatively thicker or more viscous slurry.

Typically, the azole monomer or azole copolymer is solvated in anappropriate solvent, either water, a mixture of water and misciblesolvent (ethanol, methanol, or other alcohols; acetone, tetrahydrofuran(THF), dimethylformamide (DMF), dimethylsulfoxide (DMSO), or a non-watermiscible organic solvent selected from the group including ethers, suchas dimethylether and diethylether, and also from the group includingaromatics, such as toluene and benzene. Other known additives such asslag formers, coolants, burn rate modifiers, secondary fuels, andsecondary oxidizers may be added to the solvent in known effectiveamounts to make a slurry in the solvent. The slag formers, processingaids, coolants, and/or burn rate modifiers include stearates such asmagnesium stearate, graphite, clays, micas, talcs, silicates,aluminates, and other functionally similar constituents. The secondaryfuels include tetrazoles, triazoles, imidazoles, pyrazoles,oxiadiazoles, guanidines such as nitroguanidine and guanidine nitrate,and other constituents functional as fuel components in the presentinvention. Secondary oxidizers include metal and nonmetal chlorates,perchlorates, nitrates, nitrites, oxides, and other compounds having anoxidizing function.

The polymerization initiator is then added after the addition of all ofthe other constituents, and is selected from known initiators. Forexample, a preferred free radical initiator is2,2′-azobisisobutyronitrile (AIBN) and may be employed in a knownmanner. Other types of initiators such as ammonium persulfate are alsocontemplated. In general, the polymerization initiator is provided atabout 100-250 mg per batch. Nevertheless, all that is required is thatis a relatively small amount as compared to the overall weight of themix whereby the formation of free radicals is facilitated. After that,the polymerization reaction self propagates. Temperature may beincreased or lowered to tailor the desired cure time. At roomtemperature, curing may take from three to twenty-four hours. Apreferred temperature range is from 10-90° C. Although an apparentlycured material may be obtained in a relatively short time, the curingprocess may continue for a number of hours. After mixing theconstituents to form a substantially homogeneous slurry, curing in astatic state produces a solid block of finished propellant whilestirring during the curing process forms granules.

Depending on the monomer/copolymer and the solvent temperature, 10-50%propellant mixture per unit solvent weight is desirable. The solvent canbe removed during or after the curing process by evaporation. Note thatif the monomer/copolymer is a liquid, a solvent may not be necessary. Inessence, the liquid monomer or copolymer must be in an amount effectiveto “wet” the solids to be added thereto. “Wet” as used herein is meantas at least partially solvating, and more preferably completelysolvating, the constituents added to the fuel. The effective liquidmonomer/copolymer amount can therefore be iteratively determined basedon the weight percent desired relative to the fuel function and relativeto the propensity of the fuel to wet the rest of the constituents. If asolvent is still required to wet the constituents, the solvent may beadded to ensure wetting of the solid constituents within the vessel. Ingeneral, the various constituents may be added to the slurry at thefollowing weight percents: 5-20% of the azole monomer/copolymer(s);50-90% of the oxidizer(s); 0-25% additional fuels; and 0-10% processingaids, slag formers, and/or burning rate modifiers. Note that the weightpercents represent the total weight prior to addition to the slurry, orprior to combination thereof.

Compositions formed in this manner result in consistent repeatableperformance based on the intimate combination of the constituentsresulting from the mixing and curing process. Furthermore, themanufacturing process of the gas generant is simplified as compared toother gas generant syntheses thereby reducing the associated costs. Inaddition to the advantages stated above, other advantages include theability to melt form many of the compositions when the monomer/copolymeremployed is thermoplastic in nature. Furthermore, the pliant nature ofthe compositions facilitates containment flexibility with many of thepresent compositions whereby the propellant or gas generant 12 may becompressively stored in cavities within the inflator thereby optimizingthe use of available space. As a result, the size of the inflator may beeffectively reduced while still retaining the same effective amount ofgas generation, thereby retaining the same inflation pressure profilethat would typically be represented by a relatively larger inflator.

Stoichiometric amounts of fuel and oxidizer are preferably combined inthe slurry thereby resulting in a balanced combustion reaction. Anexemplary balanced combustion reaction of poly(5-amino-1-vinyl)tetrazole with ammonium nitrate is shown below:

2C₃H₅N₅+17NH₄NO₃→6CO₂+22N₂+39H₂O

The weight percents of the fuel and oxidizer are about 14% PV5AT andabout 86% AN. Other oxidizers including strontium nitrate, potassiumperchlorate, ammonium perchlorate, and so forth may also be employeddepending on application design criteria. In general, the fuel/oxidizerweight percent ratio ranges from 45/50 to 5/90, respectively.

The polymerization process may be accelerated by the amount of initiatoremployed and also by the application of heat, for example. Otheracceleration methods are contemplated.

It is also contemplated that the present compositions be employed in anairbag device to include airbag modules, airbag inflators, seatbeltpretensioners, or, vehicle occupant restraint systems, all schematicallyrepresented in FIG. 2 and all built or designed as well known in theart. Furthermore, the present compositions may more generally beprovided in gas generating systems designed for a variety ofapplications such as inflatable flotation devices, inflatable aircraftslides, fire extinguishers, and vehicle occupant protection systems thatinclude airbag devices and/or seatbelt assemblies with pretensioners,for example.

As shown in FIG. 1, an exemplary inflator incorporates a dual chamberdesign to tailor the force of deployment an associated airbag. Ingeneral, an inflator containing a gas generant 12 formed as describedherein may be manufactured as known in the art. U.S. Pat. Nos.6,422,601, 6,805,377, 6,659,500, 6,749,219, and 6,752,421 exemplifytypical airbag inflator designs and are each incorporated herein byreference in their entirety.

Referring now to FIG. 2, the exemplary inflator 10 described above mayalso be incorporated into an airbag system 200. Airbag system 200includes at least one airbag 202 and an inflator 10 containing a gasgenerant composition 12 in accordance with the present invention,coupled to airbag 202 so as to enable fluid communication with aninterior of the airbag. Airbag system 200 may also include (or be incommunication with) a crash event sensor 210. Crash event sensor 210includes a known crash sensor algorithm that signals actuation of airbagsystem 200 via, for example, activation of airbag inflator 10 in theevent of a collision.

Referring again to FIG. 2, airbag system 200 may also be incorporatedinto a broader, more comprehensive vehicle occupant restraint system 180including additional elements such as a safety belt assembly 150. FIG. 2shows a schematic diagram of one exemplary embodiment of such arestraint system. Safety belt assembly 150 includes a safety belthousing 152 and a safety belt 100 extending from housing 152. A safetybelt retractor mechanism 154 (for example, a spring-loaded mechanism)may be coupled to an end portion of the belt. In addition, a safety beltpretensioner 156 containing propellant 12 may be coupled to beltretractor mechanism 154 to actuate the retractor mechanism in the eventof a collision. Typical seat belt retractor mechanisms which may be usedin conjunction with the safety belt embodiments of the present inventionare described in U.S. Pat. Nos. 5,743,480, 5,553,803, 5,667,161,5,451,008, 4,558,832 and 4,597,546, incorporated herein by reference.Illustrative examples of typical pretensioners with which the safetybelt embodiments of the present invention may be combined are describedin U.S. Pat. Nos. 6,505,790 and 6,419,177, incorporated herein byreference.

Safety belt assembly 150 may also include (or be in communication with)a crash event sensor 158 (for example, an inertia sensor or anaccelerometer) including a known crash sensor algorithm that signalsactuation of belt pretensioner 156 via, for example, activation of apyrotechnic igniter (not shown) incorporated into the pretensioner. U.S.Pat. Nos. 6,505,790 and 6,419,177, previously incorporated herein byreference, provide illustrative examples of pretensioners actuated insuch a manner.

It should be appreciated that safety belt assembly 150, airbag system200, and more broadly, vehicle occupant protection system 180 exemplifybut do not limit gas generating systems contemplated in accordance withthe present invention.

It will be understood that the foregoing descriptions of variousembodiments of the present invention are for illustrative purposes only,and should not be construed to limit the breadth of the presentinvention in any way. As such, the various structural and operationalfeatures disclosed herein are susceptible to a number of modifications,none of which departs from the scope of the present invention as definedin the appended claims.

1. A method of forming a gas generating composition comprising the stepsof: providing a solvent effective to dissolve a fuel comprising apolymeric azole selected from the group consisting of vinyl tetrazoles,vinyl triazoles and vinyl furazans, the solvent selected with regard tothe solvability of the functional group(s) that may be present on thepolymeric azole, the solvent placed in a mixing vessel; adding the fuelto the mixing vessel; adding an oxidizer to the mixing vessel; stirringthe mixture; adding an initiator to the mixing vessel to initiatepolymerization of the slurry; and curing the mixture.
 2. The method ofclaim 1 further comprising the step of adding an additive to the mixingvessel prior to adding the initiator.
 3. The method of claim 1 furthercomprising the step of heating the mixing vessel below boiling.
 4. Themethod of claim 1 wherein adding the solvent and adding the fuel to themixing vessel comprises the same step whereby the fuel also functions asthe solvent.